I love these threads about SLI. It has to be the most misunderstood implementation of hardware since the atomic bomb. Here's my $.02:
- SLI doesn't help much in OFF (but doesn't seem to help much, IMHO).
- SLI *DEFINITELY* does work, and it's not at all a pain - unless you consider clicking the button that says "Enable SLI" a pain. That's all I've ever really had to do, and it makes a substantial difference (except in OFF).
- The "physics" thing: This has been available for some time now. The drivers, when enabled for SLI, let you pick whether to enable or disable physics. As I understand it, enabling physics means one of the two GPUs is tasked with doing the physics work (not both).
- Some chipsets do not support running *both* PCIe slots at 16x. My opinion is you shouldn't bother with a board that won't run both at full 16x.
SLI is a mistake if you buy all new hardware, at the same time, to set it up. By that, I mean that you're better off performance wise to buy 1 high-end card than to buy two lesser cards just to set them up for SLI. (The obvious exception is those well-off enough to buy *two* high end cards, at the same time). It's pretty much long been established that you don't get 200% increase by adding a second card in SLI.
Where SLI pays off, is - assuming you already bought a SLI-capable motherboard and 1 card, then you can add another identical card later (at the point when the second card is still cheaper than the high-end card). Doing so might also forestall having to replace an entire motherboard and all that goes with (assuming your 'upgrade path' was otherwise exhausted, but the CPU still holds it's own).
Our friend Over50 writes above that he may well be the textbook case of where SLI pays off: He can increase the performance of his machine withing having to go 'full-top-end', and without having to change his entire rig out. (Note that YMMV; the performance you see is determined by lots of factors - this is one of the reasons OFF doesn't seem to benefit from SLI).
SLI is NOT the total crap almost everyone seems determined to call it. It is also NOT the end-all answer that everyone *else* seems to want to call it.
(please note the foregoing is entirely my opinion)
